1. Field of the Invention
This invention relates to an electrophotographic process.
2. Description of the Prior Art
An electrophotographic process is well known in the art in which a photosensitive member of a photoconductive member of a photoconductive material in the form of a belt or a drum moves along an arcuate path so that its peripheral surface undergoes a cyclic movement in a given direction. During such movement, it is slitwise exposed to a light image of an original to form an electrostatic latent image corresponding to the original, which latent image is developed to produce a visual image, which is in turn transferred onto a transfer medium such as paper for a subsequent fixing step.
An arrangement to carry out such an electrophotograhic process has been improved in various aspects. One of the improvements relates to an improvement in the operational efficiency i.e. an increase speed. An improvement in the operational efficiency directly translates into an increase in the number of copies which can be produced from prescribed original during a given time interval.
One of the problems which inhibits an increase in the speed of the electrophotographic process is that of a scanback. Generally, a slitwise exposure of a photosensitive member takes place as a result of a corresponding scan of the original. The original is scanned by the relative movement between the original and the exposure optics. The slitwise exposure of the photosensitive member takes place during the relative movement which occurs in a given direction. Upon completion of the slitwise exposure, the relative movement in the opposite direction must take place before the starting condition is resumed preparatory to the initiation of another slitwise exposure. The term "scan-back" refers to the reestablishment of the original positional relationship between the original and the exposure optics subsequent to the completion of the slitwise exposure. Obviously, no slitwise exposure of the photosensitive member can take place during the scan-back except where special exposure optices are employed. It then follows that, when representing the time required to effect slitwise exposure of the photosensitive member by T.sub.S and the time required for the scan-back by T.sub.R, it takes a total time of T.sub.S +T.sub.R to produce a single copy.
A speed-up of the electrophotograhic process means a reduction in the total time of T.sub.S +T.sub.R. To reduce the total time, either T.sub.S or T.sub.R must be reduced. The time T.sub.S can be reduced to a certain degree as by the exploitment of a highly sensitive photosensitive member, but in practice it is very difficult. It then follows that the time T.sub.R for the scan-back must be reduced. However, there is a limitation on this possibility, considering the inertia of movable parts which are utilized to scan the original, and consequently such reduction presents a very difficult problem.
This represents a scan-back problem, meaning the difficulty experienced in reducing the scan-back time T.sub.R. It is even more difficult if not impossible to reduce it to zero. However, instead of reducing the scan-back time T.sub.R to zero in practice, it is possible to provide an apparent reduction of the scanback time to zero in order to achieve an increase in the speed of the electrophotographic process. Specifically, a photosensitive member in the form of a belt or drum is caused to move along an arcuate path so that its peripheral surface undergoes a cyclic movement in a given direction. During such movement, the photosensitive member is subject to a slitwise exposure to a light image of an original to form a corresponding electrostatic latent image thereon. A developing step and a transfer step which transfers the resulting visual image are repeated a plurality of times upon the latent image formed. When the same latent image is utilized a plurality of times, the single exposure is sufficient to produce N.sub.T copies if the latent image is utilized a plurality of times N.sub.T. For an integer n which is defined by the inequality 1.ltoreq.n.ltoreq.N.sub.T, (N.sub.T +n) copies can be obtained by two exposures. In this instance, during the time the latent image is utilized N.sub.T times after the first exposure has been completed, the scan-back may take place, whereby the second exposure can be initiated immediately after N.sub.T copies have been produced. In other words, this represents a zero time for the scan-back, assuming the proposition that a single exposure is required for each copy produced. This is referred to herein as apparent zero time for the scan-back.
As discussed above, a speed-up of the electrophotographic operation is enabled by utilzing an apparent zero time for the scan-back in which an electrostatic latent image formed is utilized a number of times. The problem then to be solved is the manner of increasing the speed of operation of such electrophotographic process in which the latent image is utilized a number of times. A first approach to solve this problem will be the exploitment of a photosensitive member having an increased optical sensitivity. The increased sensitivity permits the time required for the slitwise exposure to form an electrostatic latent image to be reduced. However, it is not a simple matter to exploit such a photosensitive member. A second approach is to provide an increased number of segments N.sub.S of the photosensitive member. The number of segments as termed herein refers to the number of image forming regions which can be encompassed within the peripheral length of the photosensitive member.
By way of example, it may be assumed that N.sub.S =4, and N.sub.T -10. In this instance, four electrostatic latent images can be formed on the photosensitive member. Once four latent images are formed, four copies can be produced during one revolution of the photosensitive member, and hence a number of copies can be consecutively produced up to forty copies. If a certain length of time is required to form the four latent images, the photosensitive member may be subject to a high speed rotation subsequent to the formation of the latent images, so that the average time required to produce a single copy will be reduced when forty copies are to be produced, for example, thus resulting in an increased speed of operation of the electrophotographic process. However, there is another problem with this technique. The value of N.sub.T has been assumed to be equal to 10 in the example given above and such a figure for N.sub.T is fully possible if the required image quality of the copy produced is one which is sufficient to serve practical purposes. However, it is expected that the demand required on the image quality will be higher in the near future, and if the image quality of the copy should be sufficient to satisfy such demand, the limit of N.sub.T will be on the order of four at most. It will be seen that a value of N.sub.T of this order cannot result in a substantial improvement in the speed of operation.